This invention relates to a new and improved sensor activated, automatic door assembly which is attached to a building to provide support and control for the automatic opening and closing of sliding doors. The presently existing automatic door systems are provided by a small number of manufacturers who specially design and construct the components of the system to their own special specifications. As a result, the components of the various systems are not compatible or interchangeable. Experience has shown that repair and replacement of these systems is costly and time consuming. Repairs are often delayed because of dependency upon the availability of parts and service from the original manufacturer. As a result of the complexity of the systems minor repairs, servicing and replacement cannot be accomplished by the user. In addition, components of existing automatic door systems exhibit certain deficiencies and inadequacies in their design and construction which limit their installation, adjustment and repair, such as: the operator-header support beams tend to distort from the weight and operation of the system causing malfunctions and also are not adaptable to installation on the front surface of building fronts; roller track, drive belt pulley and door carrier systems are not adjustable to compensate for wear and distortion in the system caused by the continuous stress of operation and the expansion and contraction of the building resulting in unnecessary malfunctions, repairs and replacements; the operation of opening and closing the sliding doors is not adjustable; and repairs, service and replacement of components by users is complicated by non-standard replacement parts, non-availability of parts and limited accessibility to components of the system. The instant invention overcomes these deficiencies and inadequacies by providing for components and systems which are adjustable and adaptable for installation in new and existing buildings and for compensation of structural distortions. Repair, service and replacement operations by users are improved by easy access to components and availability of parts which are interchangeable and standardized throughout the industry.